Internal Displacement Reactions inmulti-component Oxides.

S.N.S.Reddy1, D.Leonard1,2, L.B.Wiggins1 and K.T.Jacob3

1 IBM CorporationSystems & Technology groupHopewell Junction, NY 12533

2 Dept. of Materials ScienceNorth Carolina State UniversityRaleigh, NC 27695

3 Dept. of MetallurgyIndian Institute of ScienceBangalore – 5600 12 , India

Introduction:

A (metal) + (B,C,…)O (oxide) = “B” (metal) + “(A,C,…)O” (oxide)

Internal displacement reactions in multi-component Oxides:

► Redox reaction inside an oxide matrix.

Related internal reactions:

► Internal displacement reaction inside a metal matrix

Nickel matrix: 3 MoO2 + 4 Cr = 2 Cr2O3 + 3 Mo

(Shook, Rapp & Hirth, Met.Trans., v.16A, 1985)

► Internal Oxidation / Reduction in a matrix

• Metal matrix: (A,B) -------------- A (matrix) + BO (ppt)( Well known in literature)

• Oxide matrix : (A,B)O ------------ AO (matrix) + B2O3 (ppt)

(A,C)2O3 ------------ A2O3 (matrix) + CO (ppt)

(A,C)O ----------------.> AO (matrix) + C (ppt)

H.Schmalzried & M.Backhaus-Riccoult,Prog.Solid St.Chem., v.22, 1993

( No published studies )

oxidation

oxidation

reduction

reduction

Internal displacement reactions:

OXIDE MATRIX :

(a) Oxide “line” compounds of narrow composition width:

A + BCOm+n = “B” + “ACOm+n”

[ Oxide line Compound ⇒ Ratio, (B:C) = (A:C) = {(A+B):C} = constant ]

(b) Oxide solid solutions of wide composition range:

x A + (BxC1-x)O = x “B” + “(AxC1-x)O”

[ x ⇒ wide range of values ]

Common features:► Cation exchange reaction (B A) and

precipitation of B in oxide matrix.

► C is “inert” for cation exchange reaction.

► No change in Oxide crystal structure.

► Concentration gradients in product phases

► Oxygen sub-lattice is rigid: Dcation >> DO

( Oxide is an electronic conductor ⇒ te ≈ 1 )

∆G0(COn) << ∆G0(AOm) < ∆G0(BOm)∆G0(ACOm+n) < ∆G0(BCOm+n)

AOm(FeO)

BOm(NiO)

(TiO2)COnCOn

+ACOm+n

COn+

BCOm+n

(A,B)COm+n+

(A,B)Om

T

COn+

(A,B)COm+n

Fig.1. Oxides system for internal displacement reactionbetween a metal and an oxide “line” compound.

ACOm+n+

AOm

(A,B)Om

BCOm+n+

BOm

A (metal) + BCOm+n (oxide) = “B” (metal) + “ACOm+n” (oxide)

Reactions in an oxide “line” compound

+Reactionpath

StartingOxide

“line” compound (A:C) = (B:C) = {(A+B):C} =constant}separate sublattice for (A,B) & C

(NiTiO3)(FeTiO3)

Displacement reaction in ilmenite structure:

Fe + NiTiO3 = “Ni” + “FeTiO3” ∆G0

1273K ≈ - 66 kJ /mole

OXIDE: Ilmenite Structure – derivative of Corundum ;Alternating sheets of Ni2+/Fe2+ and Ti4+

(two seperate cation sublattice)(Ni,Fe)TiO3 Solid Solution; Ratio, (Ni+Fe):Ti = 1:1

Fe γ-(Ni,Fe) + (Fe,Ni)TiO3 NiTiO3Reaction ZoneJFe JNi

Fe / boundary (I)High µFeTiO3High µFeLow µNiLow µO2

Reaction front (II)High µNiTiO3Low µFeHigh µNiHigh µO2

FeO NiO

TiO2

NiTiO3FeTiO3

∇µ FeTiO3 and ∇µ NiTiO3

JFe reaction frontFe/boundary JNi

∑ JTi = 0

Reaction path

( Point defects & Diffusion in Ilmenite at reaction T: No data )

NiTiO3

(Fe,Ni)TiO3+

(Ni-Fe)

Periodic precipitation of γ – (Ni,Fe) alloy ;Liesegang phenomenon {(xn+∆xn)/xn = xn+1/xn = k }?

⇒ Linear increase of spacing with band number ?

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800

Distance from Fe / reaction zone boundary, µm

Cat

ion

frac

tion

in th

e ox

ide

ReactionFront

Reaction Zone: (Fe, Ni)TiO3 NiTiO3

Ti

Fe

Ni

Ilmenite Line: (NFe + NNi ) = NTi = 0.5

Fig.10. EPMA analysis of Oxide Composition in the productzone for the reaction between Fe and single crystal NiTiO3 at 1273 K ; time = 49 hrs.(Note: EPMA points deviate from ilmenitecomposition by about 6% --due to machine calibration;the lines are drawn for eye recognition purpose only)

Internal displacement reaction in an oxide solid solution:

x A (metal) + (BxC1-x)O (oxide) = x “B” (metal) + “(AxC1-x)O” (oxide)

(A,B,C)O --- solid solution in the entire composition range.

A,B,C --- Occupy the same cation sub-lattice.

C --- “Inert” for cation exchange ; ∆G0CO << ∆G0

AO < ∆G0BO

“B” + “(AxC1-x)O”A (BxC1-x)OJA JB

∇µC = ? ; JC = ?

A / boundary (I)High µALow µBLow µO2µC = ?

Reaction front (II)Low µAHigh µBHigh µO2µC = ?

CO

AO BO

+

Reaction path?

JA = - LAA∇µA – LAB∇µB – LAC∇µCJB = - LAB∇µA – LBB∇µB – LBC∇µCJC = - LAC∇µA – LBC∇µB - LCC∇µC

At boundaries (I) & (II):µC = µCO – µO2 ; µO2 (I) < µO2 (II)

CO

AO BO

At constant T and µO2Lines of constant µCO

(C) : µCO (I) < µCO (II)however, in most cases: I ∆µO2 I > I ∆µCO I

and µC (I) > µC (II)

Net result : JC Reaction front {boundary (II)}(“up-hill” diffusion of C)

After time t : NCO at boundary (I) < (1- x) ;

NCO at boundary (II) > (1- x)

(a)(b)

(c)

(a)and (b) : µCO (I) > µCO (II)⇒ µ C (I) > µ C (II)

For a given x in the starting oxide:

A (A-B) + (A,B,C)O (BxC1-x)O

(I) (II)

Model reactions in oxide solid solutions:

x Fe + (Nix Mg1-x)O = x “Ni” + “(Fex Mg1-x)O”

x Fe + (Cox Mg1-x)O = x “Co” + “(FexMg1-x)O”

MgO

FeO NiO or CoO

(Ni,Mg)O --- “Raoultian”

(Co,Mg)O --- nearly “Raoultian”--- small positive

deviation ?

(Fe,Mg)O – positiveDeviation from “Raoultian”

+

Constant µMgO

Fe (Ni-Fe) + (Fe,Mg,Ni)O (NixMg1-x)Oor or

(Co-Fe) + (Fe,Mg,Co)O (CoxMg1-x)O

Boundary (I) Boundary (II)

µ Fe (I) > µ Fe (II)µ Mg (I) > µ Mg (II)µ Ni (I) < µ Ni (II) µ O2(I) < µO2 (II)

JFe , JMg→ ← JNi , JCo

+

“Up-hill” diffusion of Mg.

Gradient in (Fe2+ / Fe3+) ratio ⇒ effect on JFe ?

Point defect structure in Oxide: Cation Vacancies, VM = f (x , po2, T)

Fe + (Nix Mg1-x)O = “Ni” + “(Fex Mg1-x)O

T = 1273 K

Fig 9. Displacement reaction between Fe and (Co0.5Mg0.5)O at 1273 K. (a) 16 hrs; (b) 62 hrs.

Is the precipitation “periodic” for reactions in Single Crystal Oxide Solid Solutions ?

Fe + (Co0.5 Mg0.5)O = “Co” + “(Fe0.5 Mg0.5)O

T = 1273 K

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cat

ion

frac

tion

in o

xide

0 200 400 600

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

Distance from Fe/reaction zone interface, µm

NC

o of a

lloy

prec

ipita

te

Oxide Phase, (NFe + NMg + NCo) = 1

Reaction Front

Reaction Zone

Mg

Fe

Co

Alloy Phase, (NCo + NFe) = 1

(Co0.5 Mg0.5)O

Fig.9. Composition of the product phases for the internaldisplacement reaction between Fe and (Co0.5Fe0.5)O at 1273 K and 62 hrs. (Lines are for eye-recognition only)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800 1000

Distance from Fe / reaction zone boundary, µm

Cat

ion

fract

ion

in o

xide

Ti

MgFe

Ni

ReactionfrontReaction Zone

(Ni-Fe) + (Fe,Mg,Ni)TiO3

Fig.8. EPMA analysis of product oxide composition for the reactionbetween Fe and (Ni0.5Mg0.5)TiO3 .T = 1273 K ; time = 100 hrs.

Reaction in solid solutions of “line” compounds:

Fe + (Ni0.5 Mg0.5)TiO3 = “Ni” + “(Fe0.5 Mg0.5)TiO3”

( “Inert” cations : Mg & Ti )

Cation sub-lattice(i) : Ni, Mg & FeCation sub-lattice(ii) : Ti

Ilmenite structure(Fe+Ni+Mg):Ti = 1:1}

Net Cation flux:JFe , JMg reaction front; JNi Fe / boundary; JTi =0

FeTiO3 NiTiO3

MgTiO3

x

(Ni,Mg)TiO3

StartingOxide

Reaction path

(lines are for eye recognition only)

Summary(i) Oxide “line” compounds of narrow composition width:

Model reaction: Fe + NiTiO3 = “Ni” + “FeTiO3”

--- periodic precipitate of (Ni-Fe) alloy ; Gradients in NFe & NNi .

--- Product oxide, “FeTiO3” : ( FeTiO3 – NiTiO3 ) solid solution.Gradients in NFe & NNi. (Ni+Fe) : Ti = 1:1

--- Net cation flux in product oxide:

JFe reaction front ; JNi Fe / boundary ; JTi = 0.

(ii) Oxide solid solutions of wide composition range:

Model Reactions: Fe + (NixMg1-x)O = “Ni” + “(FexMg1-x)O”Fe + (CoxMg1-x)O = “Co” + “(FexMg1-x)O

--- “Ni” = (Ni-Fe) ; “Co” = (Co-Fe) ; Composition gradients.

--- “(Fex Mg1-x)O” : (Fe,Mg,Ni or Co)O solid solution.

--- Net Cation Flux: JFe , JMg reaction front ;JNi or Co Fe / boundary ;

• Internal displacement reactions are useful to synthesizeMetal-ceramic composites with unique structures.

• Only qualitative nature of diffusion in oxides can be obtained from a study of these reactions.